Cloud chamber, radiation detector, originally developed between 1896 and 1912 by the Scottish physicist C.T.R. Wilson, that has as the detecting medium a supersaturated vapour that condenses to tiny liquid droplets around ions produced by the passage of energetic charged particles, such as alpha particles, beta particles, or protons. In a Wilson cloud chamber, supersaturation is caused by the cooling induced by a sudden expansion of the saturated vapour by the motion of a piston or an elastic membrane, a process that must be repeated with each use.

In a diffusion chamber, a simpler and continuously sensitive cloud chamber, the saturated vapour is cooled to supersaturation as it diffuses into a region kept cold by a coolant such as solid carbon dioxide or liquid helium.